保德地区煤层气地球化学特征及成因探讨

为了查明保德地区煤层气地球化学特征及成因，采集煤样、煤层气样及水样，开展气体组分分析、煤层气井产出水水质检测和稳定同位素分析。结果表明:煤层气组成中烃类气体以CH₄为主，体积分数为88.60%~97.59%；含有少量乙烷，体积分数仅为0.01%~0.14%；干燥系数均大于0.99，属于极干煤层气。非烃类组分中，主要含有CO₂和N₂，其中，CO₂体积分数为1.74%~7.61%，N₂体积分数为0.04%~8.18%。煤层气δ13C(CH₄)值为–56.8‰~–47.7‰，δ13C(CO₂)值为–6.6‰~13.9‰，δD(CH₄)值为–252.6‰~–241.6‰。煤层产出水呈弱碱性，属于NaHCO₃类型水，与地表水离子构成、矿化度、δD(H₂O)和δ18O(H₂O)值均相近，有地表水的补给，有利于产CH₄菌大量繁殖，生成次生生物气。综合认为，研究区煤层气为热成因气和生物气的混合气，生物成因气主要是通过二氧化碳还原作用形成，受煤层解吸–扩散–运移作用、水溶作用和次生生物作用导致煤层气“变轻”。研究成果为后续煤层气勘探开发提供指导。 

Geochemical characteristics and genesis of coalbed methane in Baode area

In order to find out the geochemical characteristics and origins of coalbed methane(CBM)in Baode area, coal, CBM and water samples were collected, gas component analysis, quality detection of CBM well-produced water and stable isotope analysis were carried out. The results show that CH₄ is the main hydrocarbon gas in CBM composition and its volume fraction ranges from 88.60% to 97.59%; a small amount of ethane is contained, and the volume fraction is only 0.01%-0.14%. The drying coefficient is greater than 0.99, the CBM belongs to the extremely dry CBM. The non-hydrocarbon components mainly contain CO₂ and N₂, of which the CO₂ volume fraction ranges from 1.74% to 7.61% and N₂ volume fraction ranges from 0.04% to 8.18%. CBM δ13C(CH₄) value is between –56.8‰ and –47.7‰, the δ13C(CO₂) value lies between –6.6‰ and 13.9‰, the δD(CH₄) value is between –252.6‰ and –241.6‰.The water produced from coal seam is weakly alkaline, belonging to NaHCO₃ type water, which is similar to the ion composition, salinity, value of δD(H₂O) and δ18O(H₂O) of surface water, and the recharge from surface water is conducive to the proliferation of CH₄-producing bacteria and the formation of secondary biogas. It is generally believed that CBM in the study area is a mixture of thermal and biogenic gas, formed mainly through carbon dioxide reduction, and “lightened” due to coal seam desorption, diffusion and migration, water dissolution and secondary biological action. The research results provide guidance for subsequent CBM exploration and development.